libct: Signal: honor RootlessCgroups #6520

Job	Run time
cross-i386	2m 22s
cross-i386 (runc_nodmz)	1m 55s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 25s
test (ubuntu-20.04, 1.22.x, rootless)	7m 3s
test (ubuntu-20.04, 1.22.x, -race)	8m 29s
test (ubuntu-20.04, 1.22.x)	7m 23s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 19s
test (ubuntu-20.04, 1.23.x, rootless)	7m 29s
test (ubuntu-20.04, 1.23.x, -race)	8m 8s
test (ubuntu-20.04, 1.23.x)	7m 31s
test (ubuntu-20.04, 1.23.x, criu-dev)	8m 9s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 12s
test (ubuntu-24.04, 1.22.x, rootless)	9m 4s
test (ubuntu-24.04, 1.22.x, -race)	8m 3s
test (ubuntu-24.04, 1.22.x)	7m 31s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 29s
test (ubuntu-24.04, 1.23.x, rootless)	9m 31s
test (ubuntu-24.04, 1.23.x, -race)	8m 8s
test (ubuntu-24.04, 1.23.x)	7m 25s
test (ubuntu-24.04, 1.23.x, runc_nodmz)	7m 37s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 37s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 41s
test (actuated-arm64-6cpu-8gb, 1.23.x)	8m 39s
all-done	1s
	2h 58m 11s

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